building a 10mA current source

Discussion in 'General Electronics Chat' started by gehan_s, Oct 3, 2012.

  1. gehan_s

    Thread Starter Active Member

    Sep 17, 2012
    hey all,

    i need to build a current source where i have to draw 10mA from a 1.25V to 20V varying DC supply. my question is will this work for the given ranges and if the voltage at the collector becomes 0V or at 20V will it destroy the transistor ???. this is what i have come up with

    thanks in advance !!!!!!!!!!!!!!
  2. JDT

    Well-Known Member

    Feb 12, 2009
    That should work.

    No you won't destroy the transistor with 0V on the collector (or open circuit).

    For the high voltage check transistor max collector volts (probably any transistor OK for 20V) and maximum power dissipation (only 0.2W in your case so OK).

    But if you were designing for higher currents or higher voltages you would also need to check the "Safe Operating Area" from the transistor data sheet.

    Don't forget that the transistor base current is also "measured" by the emitter resistor so for better accuracy I suggest a "darlington" pair of transistors should be used. (Can be got in a single package). Or better, a MOSFET.
    gehan_s likes this.
  3. bertus


    Apr 5, 2008

    The 25 Volts for the Vceo might be a bit short if you want to run it upto 20 Volts.
    It might stress the transistor.
    For safety take a transistor with a Vceo of at least 40 Volts.

    gehan_s likes this.
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    It should "work," but it is regulating the base + collector current, not the collector current. A mosfet instead of a transistor would regulate drain current alone.
    gehan_s likes this.
  5. gehan_s

    Thread Starter Active Member

    Sep 17, 2012
    thank you all for your replies. they helped me a lot !!!!!!!!!!!!!!!!
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    You will have about 200 mW of power dissipation maximum. If it's a TO-92 device, it will raise the temp about 40C above ambient. A little hot but not dangerous.

    Just don't use any of those microscopic tiny surface mount devices or they might cook too hot.
    gehan_s likes this.